1. Field of the Invention
The present invention relates generally to the orienting of objects and more specifically to the orienting of arrays of solar panels, suspended by cables from a balloon gondola.
2. Description of the Related Art
Many devices are designed to control two tilt degrees of freedom (dofs) to properly orient an object. Some orienting devices directly pivot the object using rotary actuators while others use two linear actuators in conjunction with some kind of pivot point. Many include a stabilizing mechanism to prevent rotation about a third axis.
One example of an orientation control device is used in security cameras. Some security cameras control two rotations directly. Such a camera may have a tip-tilt mechanism to aim the camera up and down with the tip-tilt mechanism mounted on a pan arrangement that allows the camera to be aimed left and right. In this construction, the camera cannot rotate about its optical axis (i.e., the line of sight) so that the top of the camera always corresponds to the top of the image. Thus, two dofs are controlled while the third rotational dof is constrained. Such an orienting device may be referred to as an azimuth/elevation mechanism since azimuth and elevation angles are controlled independently.
Due to the inherent constraint on the third degree of freedom, use of an azimuth/elevation mechanism to orient an object toward a target may hinder the intended operation and cause the designer to add a third (possibly unnecessary) actuator or other compensating mechanism. For example, when an azimuth/elevation device is mounted on a vehicle, the azimuthal control may require several revolutions about the azimuthal axis, which is usually vertical, if the vehicle makes several turns. Likewise, if the vehicle is stationary and the target moves around the vehicle, the azimuthal control may also require multiple revolutions. In these situations, any cables running from the vehicle to the device will become twisted. To avoid this, the designer may choose to use slip rings for electrical connections or some kind of rotary joint for fluid lines. However these design compromises may be unnecessary if the device does not require this constraint on the third degree of freedom.
Many devices exist for which rotation about the third axis (i.e., the line of sight) is not important and need not be constrained. For example, a flat solar panel aimed directly at the sun will still generate the same amount of power even if the panel is rotated about the line from the sun to the panel. Similarly, a flat mirror can be rotated about the axis perpendicular to its surface while the reflection remains undistorted in the plane of the mirror. Under these conditions, it is necessary only to control the two tilt dofs while the third dof need not be constrained. Even so, objects such as solar panels and mirrors are often mounted on azimuth/elevation mechanisms, presumably because these mechanisms are readily available.
For example, solar panels are sometimes included in some scientific balloon payloads. Predictably, the balloon and its gondola will tend to rotate slowly about the vertical due to small air currents. The solar panels should be pointed directly towards the sun. However, if an azimuth/elevation mechanism is used, then as the gondola slowly rotates, the cables connecting the solar panel to the balloon will become twisted. As discussed above, various compromises can be used to deal with this situation. Although slip rings may be incorporated to allow the continuous rotation, these are susceptible to dirt, which can cause unreliable electrical contact. Some other means of preventing the rotation of the entire payload (i.e., gondola) may be added, such as a reaction wheel. Alternatively, the cable can be unwound occasionally by reversing the azimuthal drive. However, this operational complexity requires monitoring the number of accumulated revolutions. Also, target-lock cannot be maintained during this procedure.
Orienting a telescope suspended from a balloon leads to similar difficulties. Unlike a security camera, telescope rotation about an optical axis is not of significant concern. Although there may be some advantage to having the same edge of a target, such as a star, at the top of the image, any benefits can be traded off against the mechanical complexity associated with this constraint.
Likewise, a radar dish needs to be aimed in two directions, but rotation about the line of sight is usually unimportant. This device performs the same regardless of roll about the direction of transmission.
Orienting arrangements that avoid azimuth/elevation mechanism constraints have also been developed. For example, a tip/tilt mechanism such as the security camera tip tilt mechanism described above can alternately control two angular dofs by using two linear dof actuators to push or pull at the edges of the device being oriented. As an example, a type of mirror mount used with laser optics in laboratories is in FIG. 16. From FIG. 16, two micrometers or thumbscrews push against two corners of the mount while a third corner is held against a ball bearing acting as a pivot. Retention springs are used to maintain contact at these three points. A mirror fastened to such a mount can be aimed up/down and left/right. For these devices, the angular range of motion may be just a few degrees, but the concept can be extended to a much greater angular range.
Another example of orientation control is the swash plate of a helicopter main rotor. This device transfers the collective and cyclic controls from the pilot's control yoke across the rotating boundary to the moving main rotor. Such a swash plate arrangement includes two push rods with ball and socket end joints to tilt the swash plate about a central pivot and converts two linear dofs into two tilt dofs by means of a pivot bearing or hinge.
An even simpler example is a common venetian blind. A venetian blind controls tilt about a single axis by shortening and lengthening support cables. This device also maintains the general parallelism of many suspended articles (i.e., the blinds) by the parallelogram-like arrangement of the supporting cables. Further, the stack of suspended articles are stowed and extended by using cables to raise and lower the articles in the suspended stack. A venetian blind also uses the weight of the suspended articles to draw the supporting cables out through their guides during deployment and correspondingly to draw the cables back during retraction.
Ineffective mechanisms for deployment and retraction have limited the effective re-use of components in some relevant applications regarding the orienting of solar panels. For example, some scientific balloon systems use solar panels to provide power, where the panels are usually mounted in a configuration prior to launch. Often, due to the need to minimize weight, the panels are somewhat flimsy and delicate. Typically, there are two panels, one extending from side of the gondola. While these panels survive launch, there is usually no attempt to protect them during cut-down and landing; they are thus considered "disposable." During cutdown, the balloon tether is severed and a parachute is used to control the descent of the gondola. The opening shock when the parachute inflates can be as high as ten gravities. Similarly, there is significant deceleration when the gondola lands. Despite energy-absorbing material on its bottom and edges, the gondola experiences significant shock loading on impact. Without an effective deployment and retraction system for the panels, suitable protection to guarantee re-use appears unlikely.